In a number of medical procedures, it is desired to obtain an indication of the concentration of carbon dioxide in the exhaled breath of a patient. This determination, generally known as capnography, is typically conducted using an infrared analysis device. Such devices typically require that the gas being monitored be substantially dry, i,e., free from condensed water or other liquid constituents. Because exhaled gases typically exit a patient at about 100% relative humidity, as the gas cools during travel through the sampling line and equipment, moisture condenses. It is important to remove this moisture from the gas sample which is typically continuously drawn from the exhaled gases, usually at a flow rate of about 200 ml per minute or less.
Because, in capnography, it is desired to obtain continuous and instantaneous indications of the constitution of the exhaled gases, it is also important to avoid substantial mixing of gases sampled at different times. Accordingly, to avoid dampening the capnographic signal, the internal volume of the sampling line, including the volume of any gas/liquid separators, is preferably kept as small as possible. For this reason, it is generally desirable that a gas/liquid separating apparatus employ a separating chamber having a microvolume, i.e., a volume less than about 1 ml, and preferably much smaller.
Several approaches to solving these problems have been tried. U.S. Pat. No. 4,304,578 issued Dec. 8, 1981 to Hakala, et al. discloses a water separator which includes a downward tapering conical separation chamber. U.S. Pat. No. 4,382,806 issued May 10, 1983 to Hakala, et al. discloses a separation chamber with straight sides for precipitation of condensed water and a downward tapering portion at the lower end. U.S. Pat. Nos. 4,579,568 issued Apr. 1, 1986 to Ricciardelli, et al. and 4,592,368 issued June 3, 1986 to Ricciardelli, et al. disclose a separation chamber with a barrier which prevents direct flow to a second chamber that diverges upwardly. U.S. Pat. No. 4,717,403 issued Jan. 5, 1988 to Choksi discloses an apparatus which uses centrifugal separation of the gas and condensed moisture.
Of the above-described approaches, none describes making use of capillary forces or methods for No. 4,703,095 issued Dec. 15, 1987 to Ricciardelli discloses a separator with downwardly tapering pyramid-shaped lower walls forming internal corners with angles that produce capillary action for use in separation of a liquid from a gas. The capillary internal corners converge towards the liquid port to channel liquid away from the gas outlet port. Because of the downward tapering configuration of the separation chamber, a smaller interior wall surface area is available for formation of capillary volumes toward the bottom of the separation chamber. The total capillary volume in the lower area of the separation chamber thus is less than that in the upper region of the separation chamber. Accordingly, it would be useful to provide a separation configuration in which more surface area is available for capillary action in the lower portions of the separation chamber, without sacrificing effectiveness or efficiency of the capillary action.
Previously available separators of this type also have been subject to other disadvantages. Typical separators withdraw some amount of gas along with the separated liquid which is, therefore, not transmitted to the gas analysis device. This so-called blow down gas is preferably minimized because blow down gas is not available to the gas analyzer. Although previous devices are disclosed as requiring as little as 2 to 5% blow down for water separation, in practice it has been found that separation of liquids with viscosity higher than water (such as typically occurs when bodily fluids are mixed in the exhaled gases) require an increase in blow down ratio, occasionally greater than 15 to 20%. Accordingly, it would be useful to provide an apparatus with sufficient separation efficiency that the blow down ratio can be maintained less than about 15% even when the viscosity of the liquid is greater than that of water.
As noted above, it is desirable, in order to avoid damping or otherwise distorting the capnographic signal, that the separation chamber be kept at a small volume. Accordingly, it is desirable to provide a separation chamber with means for reducing the volume occupied by gas by substantial amounts such as 0.04 ml or more, to provide a dead space having a volume of about 0.1 ml or less.
Previous devices typically rely exclusively on gravitational means for separation or for channelling separated liquid away from the dry gas outlet port. Accordingly, previous devices are unusable when tilted substantially out of the preferred operating orientation or in reduced or zero gravity conditions. Accordingly, it would be useful to provide a device which will operate when tilted in positions up to 90.degree. from the vertical or in a reduced or zero gravity environment.
Relatedly, previous devices, if tipped out of the preferred orientation, will permit liquid to exit from the dry gas outlet port causing fouling of the gas analyzer and consequent expensive cleaning or repairs. Accordingly, it would be useful to provide an apparatus in which tilting at greater than a predetermined angle results in prevention of the passage of liquid through the dry gas outlet port.
Because of the time and expense involved in sterilizing medical equipment, it would be also be advantageous to provide a device in which the regions which might become contaminated with the separated liquid are detachable from the remainder of the apparatus and are constructed using sufficiently inexpensive procedures and materials that such portions can be disposed of rather than cleaned and reused.